docs: def _get_reciprocal(self):
          """Return the :class:`Arrow` that connects my origin and destination
          in the opposite direction, if it exists.
  
          """
          orign = self.portal['origin']
          destn = self.portal['destination']
          if (
                  destn in self.board.arrow and
                  orign in self.board.arrow[destn]
          ):
              return self.board.arrow[destn][orign]
          else:
              return None